Overweight and obesity have reached epidemic proportions in the United States. As few behavioral treatments are effective for weight loss, bariatric surgery is an increasingly important option. Although these procedures are more helpful than psychosocial treatments, post-operative weight losses vary widely and a notable subset of patients demonstrate suboptimal weight loss or substantial weight regain. Adherence to treatment recommendations is a likely contributor to these variable outcomes. Smartphone applications (apps) designed to augment behavior change or enhance adherence, have expanded exponentially in recent years, and are a potentially useful option to improve outcomes, but rigorous research on apps is limited, and high-quality, adequately powered, randomized controlled trials with large samples are required. This Fast Track study (Phase I/II) application will test a combined smartphone app and health coaching system to improve adherence, and behavioral/psychosocial outcomes for patients receiving bariatric surgery. Specifically, Phase I will utilize a small pre-surgery randomized trial (n=50) of a coach-led mobile application for bariatric candidates (Noom Coach for Bariatric Health), to establish the initial efficacy of this app by comparing the use of 8 weeks of use (n=25) to standard pre-surgery treatment (n=25). This phase will establish within-subject effects for change in diet, exercise, and psychosocial health; estimate between subject effect sizes between low intensity (no treatment) and high intensity treatment (group-based Noom Coach Bariatric Health); and validate local and global adherence algorithms for Noom Coach participants. If Noom Coach for Bariatric Health does not produce benefits for participants in this phase of the study, Phase II will not be initiated. Phase II would compare usual pre-operative care and pre-surgical use of Noom Coach for Bariatric Health on post-surgery adherence and psychosocial variables in a larger test (n=200) to explore whether usage of the app is a viable predictor of subsequent post-operative weight trajectories 12 months following surgery. We hypothesize that app users will demonstrate greater adherence to diet and physical activity recommendations and better psychosocial adjustment following surgery. In addition, Phase II will use a sequential multiple assignment randomized trial (SMART) design to determine whether Noom Coach for Bariatric Health can augment weight loss post-surgery based on weight loss from 3-6 months. Data will also be collected in Phase II to examine app usage as a viable predictor of subsequent post-operative weight and psychosocial trajectories in the 12 months post-surgery and establish sensitivity and specificity of early optimal response to surgical intervention using anthropometric (weight loss), diet and exercise adherence, and psychosocial adjustment. If the efficacy of Noom Coach for Bariatric Health is demonstrated, critical information will be available for developing a Phase III sequential multi-phase adaptive trial aimed at informing algorithms for integration into Noom Coach more broadly, and maximizing value within health care systems.